Similar to a peabrain but with a much greater measure of stupidity; only a true retard could be a pissbrain.

May also be used to describe some asshaberdasher who actually urinates into his brain by connecting a small-diameter hose from their ding-a-ling (or peehole) to one of their ears (probably one with a busted tympanic membrane {eardrum}) and then passing micturition through rhe tube; thus filling their cranium with piss..

Geez, look at Horhay over there attempting to force a triangular peg through a circular opening! What a fucking pissbrain!